1. Field of the Invention
The present invention relates to power supply noise measuring circuit and power supply noise measuring method, which are capable of evaluating a frequency component of power supply noise by using a cross-correlation function.
2. Description of the Related Art
In recent years, fluctuation in an amount of a current consumed by a microprocessor is increasing due to a rise in its operating frequency and to an increase in the number of mounted transistors and, as a result, power supply noise tends to increase. The increase of power supply noises causes various adverse effects such as an increase in an amount of signal delay in a critical path of an LSI (Large Scale Integration circuit), a decrease in a transmission margin of a high-speed I/O (Input/Output) circuit caused by increased clock jitters, or a like and, in the worst case, causes malfunctions of a circuit and there is a fear that an LSI does not function properly.
In order to avoid such a situation as above, conventionally, a method is employed, as a means to reduce noise, in which a decoupling capacitor is mounted within an LSI and a charge is supplied from the decoupling capacitor immediately when a change occurs in a generated power supply voltage to suppress the change in the power supply voltage.
In circuit designing as a countermeasure against noise, LSI chips are first produced based on an analysis on the estimation of noise and, if the countermeasure proves to be insufficient, the flow of processes is controlled so that the noise estimation result is fed back again to the stage of the LSI designing. However, in the estimation of an amount of noise, when a frequency of noise is low, if noise is measured outside the LSI (in the LSI package or on the mounting board), a problem related to measurement accuracy does not arise. In recent years, however, as a frequency of noise becomes higher, the noise measurement outside the LSI has become insufficient in terms of accuracy. Moreover, due to a decrease in a design margin occurring as an operating frequency of an LSI chip becomes higher, if a change in a power supply voltage is factored into a margin in logic delay designing in LSIs, some cases appear in which logic design does not converge in a critical path.
To achieve circuit designing with a countermeasure against noise, measurement of noise is required and, when power supply noise is to be measured outside an LSI, it is impossible to probe a high-frequency component of noise with high accuracy and, therefore, in many cases, the measurement is made by using a blunt waveform of noise. Moreover, when power supply noise is to be measured inside LSIs, many measuring methods using a circuit assembled by directing attention to a magnitude of noise are disclosed, however, only the magnitude of noise cannot be used as a direct index of indicating how noise affects performance of the LSI.
In the circuits shown in FIGS. 9A and 9B disclosed by Ali Muhtaroglu et. al of INTEL at the VLSI symposium 2003, a rough waveform of noise is produced within LSIs and, as shown in FIG. 10, an amount of noise is measured. However, there is a report by Tawfik Rahal-Arrabi of INTEL at the VLSI symposium 2005 that, even if an amount of noise increased, no great difference in the maximum operating frequency of an LSI was found (see FIG. 11). That is, a learned society's knowledge that noise amounts are not directly related to degradation of LSI performance is generally spreading.
In the circuits shown in FIGS. 9A and 9B, by applying a reference current from the Reference Unit shown in FIG. 9A, throwing-off of a balance between I ref+ and I ref− is monitored by the Detector Module shown in FIG. 9B and a differential of a shift is digitized to be outputted. FIG. 10 shows outputs from the circuits shown in FIGS. 9A and 9B and fluctuations on the Vcc side are detected on the Vref1 side and fluctuations on the Vss are detected on the Vref2. That is, fluctuations of Vcc (Iref+) and Vss (Iref−) are seen from the reference voltage side. Moreover, in FIG. 10, its upper part shows a profile of a power supply and a profile of GND and its lower part shows results from measurement by using the INTEL circuit shown in FIGS. 9A and 9B.
Moreover, FIGS. 11A and 11B show the evaluation of results from a simulation by assembling specified circuits in INTEL for logic signal delay in an LSI. FIG. 11A shows an example in which, between a case of the connection of an RC (Resistor-Capacitor) filter to separate a global clock power supply from a core clock power supply in a pseudo manner and a case of no connection of the filter, a comparison as to whether or not the connection of the filter affects performance of a data path (logic) is made by performing a simulation. The example is based on a prediction that, if a state occurs in which a clock power supply is affected by the core logic power supply, since fluctuations of the core logic power supply are larger than those of the clock power supply, a time fluctuation of a global clock becomes larger, which would affect logic delay. Also, FIG. 11B shows an effect on a data path when an RC filter is removed and also shows, since data transfer is made impossible unless an operating frequency is lowered, a result from the checking of a degree of performance degradation occurring when the operating frequency is lowered. The result shows that degradation of performance is about several percentages at most and, therefore, this result cannot be explained well unless a discussion is made after a frequency of noise is exactly understood.
In addition to the above, in Patent Reference 1 (Japanese Patent Application Laid-open No. 2001-051065, FIG. 1, Paragraph [0015]), a lightning noise identifying method at a time of observing real time data is disclosed in which real time data is observed and a cross-correlation value is calculated among observed components in every observation step and the calculated cross-correlation value is compared with a threshold and, when the cross-correlation value exceeds a threshold value, it is judged that a lightning noise is contained in the observed real time data.
Thus, according to the Patent Reference 1, at the time of observing real time data, even if a lightning noise is contained in observed data, identification between an original observed data and a lightning noise is made possible. However, in the Patent Reference 1, there is no description of technology that, by evaluating a frequency component of power supply noise using a cross-correlation function, noise affecting performance of an LSI can be evaluated within the LSI.
Also, power supply noise measuring device is disclosed in Patent Reference 2 (Japanese Patent Application Laid-open No. 2005-249408, Paragraph [0030]) in which a signal fed from power supply line is passed through a HPF (High Pass Filter) and a divided voltage is added to the signal to generate the first signal and the second signal obtained by adding a divided signal to an identified voltage and a comparator outputs a result from comparison between a voltage of the first signal and a voltage of the second signal and a counter counts up voltages when a voltage of the first signal is a voltage of the second signal or more and a sample holding circuit sample-holds a count value immediately before resetting of a counter.
Thus, according to the power supply noise measuring device disclosed in Patent Reference 2, an amount of power supply noise with a high frequency can be measured on chips (on an LSI) and sudden peak noise can be obtained and its output is a rapid signal and can be taken out to the outside. However, in the Patent Reference 2, there is no description of technology that, by evaluating a frequency component of power supply noise using a cross-correlation function, noise affecting performance of an LSI can be evaluated within the LSI.
A flow measuring method and a flow measuring device are disclosed in Patent Reference 3 (Japanese Patent Application Laid-open No. Sho 60-034430, Figures, left upper column L11 to right upper L11 on page 6) for measuring a flow of a fluid between an input point of a system and an output point of a system, the method of which includes a process of adding an indicated physical amount of a known stochastic exciting signal to a fluid at the input point of the system, a process of obtaining a response signal by detecting a fluid response depending on time with respect to the exciting signal at the output point of the system, a process of obtaining a cross-correlation function by cross-correlating the exciting signal with the response signal, a process of taking out a signal representing a flow rate from information about an indicated physical amount of a cross correlating signal and exciting signal, and a process of feeding the signal representing the flow to a signal using unit.
Thus, according to the technology disclosed in Patent Reference 3, by adding a stochastic exciting signal to the input of the system to generate a measurable output signal at an exit on a downstream side of the system, a uniform flow like a blood in particular is measured and by analyzing cross-correlation between an exciting signal and an output signal, a flow rate can be calculated. However, in the Patent Reference 3, there is no description of technology that, by evaluating a frequency component of power supply noise using a cross-correlation function, noise affecting performance of an LSI can be evaluated within the LSI.
Moreover, a spectrum dispersion transmission system and signal receiving device are disclosed in Patent Reference 4 (Japanese Patent Application Laid-open No. Hei 09-083582, paragraphs [0006] and [0038], formula 2), the signal receiving device of which includes a mixer to get a product obtained by multiplying a sine wave and cosine wave taken out from a reference carrier wave being synchronized with a transmission-side carrier wave by a receiving wave, a matched filter to realize correlation by multiplying a low frequency component of the sine wave and cosine wave output from the mixer by standby I, Q patterns produced from pseudo noise signal systems each having the same period and different phase, and a phase detector to obtain a demodulated output from the matched filter output and, the transmission device of which includes a pattern generator containing a pattern to realize phase transition by the standby I, Q patterns generated from the pseudo noise signal system on the signal-receiving side, a switching device to perform switching between a digital signal for transmission and a pattern generator output, a modulator to modulate an output from the switching device.
Thus, unlike standby patterns of the related receiving device which are not PN (Pseudo Noise) system causing a high probability of erroneous synchronization, there is a low probability of erroneous synchronization in the transmitting and receiving devices disclosed in Patent Reference 4. However, in the Patent Reference 4, there is no description of technology that, by evaluating a frequency component of power supply noise using a cross-correlation function, noise affecting performance of an LSI can be evaluated within the LSI.
The problem is that the learned society's knowledge that noise amounts calculated by the related power supply noise measuring circuit and measuring method are not directly related to degradation of LSI performance is generally spreading.